QUIC M. Thomson
Internet-Draft Mozilla
Intended status: Standards Track July 10, 2019
Expires: January 11, 2020
Version-Independent Properties of QUICdraft-ietf-quic-invariants-06
Abstract
This document defines the properties of the QUIC transport protocol
that are expected to remain unchanged over time as new versions of
the protocol are developed.
Note to Readers
Discussion of this draft takes place on the QUIC working group
mailing list (quic@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/search/?email_list=quic [1].
Working Group information can be found at https://github.com/quicwg
[2]; source code and issues list for this draft can be found at
https://github.com/quicwg/base-drafts/labels/-invariants [3].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 11, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Internet-Draft QUIC Invariants July 2019Appendix A is a non-exhaustive list of some incorrect assumptions
that might be made based on knowledge of QUIC version 1; these do not
apply to every version of QUIC.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document uses terms and notational conventions from
[QUIC-TRANSPORT].
3. An Extremely Abstract Description of QUIC
QUIC is a connection-oriented protocol between two endpoints. Those
endpoints exchange UDP datagrams. These UDP datagrams contain QUIC
packets. QUIC endpoints use QUIC packets to establish a QUIC
connection, which is shared protocol state between those endpoints.
4. QUIC Packet Headers
A QUIC packet is the content of the UDP datagrams exchanged by QUIC
endpoints. This document describes the contents of those datagrams.
QUIC defines two types of packet header: long and short. Packets
with long headers are identified by the most significant bit of the
first byte being set; packets with a short header have that bit
cleared.
Aside from the values described here, the payload of QUIC packets is
version-specific and of arbitrary length.
4.1. Long Header
Long headers take the form described in Figure 1. Bits that have
version-specific semantics are marked with an X.
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Internet-Draft QUIC Invariants July 2019
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
|1|X X X X X X X|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DCID Len (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Connection ID (0..2040) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SCID Len (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Connection ID (0..2040) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: QUIC Long Header
A QUIC packet with a long header has the high bit of the first byte
set to 1. All other bits in that byte are version specific.
The next four bytes include a 32-bit Version field (see Section 4.4).
The next byte contains the length in bytes of the two Connection IDs
(see Section 4.3) that follow. Each length is encoded as a 4-bit
unsigned integer. The length of the Destination Connection ID (DCIL)
occupies the high bits of the byte and the length of the Source
Connection ID (SCIL) occupies the low bits of the byte. An encoded
length of 0 indicates that the connection ID is also 0 bytes in
length. Non-zero encoded lengths are increased by 3 to get the full
length of the connection ID; the final value is therefore either 0 or
between 4 and 18 bytes in length (inclusive). For example, a byte
with the value 0xe0 describes a 17 byte Destination Connection ID and
a zero byte Source Connection ID.
The connection ID lengths are followed by two connection IDs. The
connection ID associated with the recipient of the packet (the
Destination Connection ID) is followed by the connection ID
associated with the sender of the packet (the Source Connection ID).
The remainder of the packet contains version-specific content.
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Internet-Draft QUIC Invariants July 20194.2. Short Header
Short headers take the form described in Figure 2. Bits that have
version-specific semantics are marked with an X.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
|0|X X X X X X X|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Connection ID (*) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: QUIC Short Header
A QUIC packet with a short header has the high bit of the first byte
set to 0.
A QUIC packet with a short header includes a Destination Connection
ID. The short header does not include the Connection ID Lengths,
Source Connection ID, or Version fields.
The remainder of the packet has version-specific semantics.
4.3. Connection ID
A connection ID is an opaque field of arbitrary length.
The primary function of a connection ID is to ensure that changes in
addressing at lower protocol layers (UDP, IP, and below) don't cause
packets for a QUIC connection to be delivered to the wrong endpoint.
The connection ID is used by endpoints and the intermediaries that
support them to ensure that each QUIC packet can be delivered to the
correct instance of an endpoint. At the endpoint, the connection ID
is used to identify which QUIC connection the packet is intended for.
The connection ID is chosen by each endpoint using version-specific
methods. Packets for the same QUIC connection might use different
connection ID values.
4.4. Version
QUIC versions are identified with a 32-bit integer, encoded in
network byte order. Version 0 is reserved for version negotiation
(see Section 5). All other version numbers are potentially valid.
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The properties described in this document apply to all versions of
QUIC. A protocol that does not conform to the properties described
in this document is not QUIC. Future documents might describe
additional properties which apply to a specific QUIC version, or to a
range of QUIC versions.
5. Version Negotiation
A QUIC endpoint that receives a packet with a long header and a
version it either does not understand or does not support might send
a Version Negotiation packet in response. Packets with a short
header do not trigger version negotiation.
A Version Negotiation packet sets the high bit of the first byte, and
thus it conforms with the format of a packet with a long header as
defined in Section 4.1. A Version Negotiation packet is identifiable
as such by the Version field, which is set to 0x00000000.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
|1|X X X X X X X|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version (32) = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DCID Len (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Connection ID (0..2040) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SCID Len (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Connection ID (0..2040) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Supported Version 1 (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Supported Version 2 (32)] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Supported Version N (32)] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Version Negotiation Packet
The Version Negotiation packet contains a list of Supported Version
fields, each identifying a version that the endpoint sending the
packet supports. The Supported Version fields follow the Version
field. A Version Negotiation packet contains no other fields. An
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endpoint MUST ignore a packet that contains no Supported Version
fields, or a truncated Supported Version.
Version Negotiation packets do not use integrity or confidentiality
protection. A specific QUIC version might authenticate the packet as
part of its connection establishment process.
An endpoint MUST include the value from the Source Connection ID
field of the packet it receives in the Destination Connection ID
field. The value for Source Connection ID MUST be copied from the
Destination Connection ID of the received packet, which is initially
randomly selected by a client. Echoing both connection IDs gives
clients some assurance that the server received the packet and that
the Version Negotiation packet was not generated by an off-path
attacker.
An endpoint that receives a Version Negotiation packet might change
the version that it decides to use for subsequent packets. The
conditions under which an endpoint changes QUIC version will depend
on the version of QUIC that it chooses.
See [QUIC-TRANSPORT] for a more thorough description of how an
endpoint that supports QUIC version 1 generates and consumes a
Version Negotiation packet.
6. Security and Privacy Considerations
It is possible that middleboxes could use traits of a specific
version of QUIC and assume that when other versions of QUIC exhibit
similar traits the same underlying semantic is being expressed.
There are potentially many such traits (see Appendix A). Some effort
has been made to either eliminate or obscure some observable traits
in QUIC version 1, but many of these remain. Other QUIC versions
might make different design decisions and so exhibit different
traits.
The QUIC version number does not appear in all QUIC packets, which
means that reliably extracting information from a flow based on
version-specific traits requires that middleboxes retain state for
every connection ID they see.
The Version Negotiation packet described in this document is not
integrity-protected; it only has modest protection against insertion
by off-path attackers. QUIC versions MUST define a mechanism that
authenticates the values it contains.
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This section lists a sampling of incorrect assumptions that might be
made based on knowledge of QUIC version 1. Some of these statements
are not even true for QUIC version 1. This is not an exhaustive
list, it is intended to be illustrative only.
The following statements are NOT guaranteed to be true for every QUIC
version:
o QUIC uses TLS [QUIC-TLS] and some TLS messages are visible on the
wire
o QUIC long headers are only exchanged during connection
establishment
o Every flow on a given 5-tuple will include a connection
establishment phase
o The first packets exchanged on a flow use the long header
o QUIC forbids acknowledgments of packets that only contain ACK
frames, therefore the last packet before a long period of
quiescence might be assumed to contain an acknowledgment
o QUIC uses an AEAD (AEAD_AES_128_GCM [RFC5116]) to protect the
packets it exchanges during connection establishment
o QUIC packet numbers appear after the Version field
o QUIC packet numbers increase by one for every packet sent
o QUIC has a minimum size for the first handshake packet sent by a
client
o QUIC stipulates that a client speaks first
o A QUIC Version Negotiation packet is only sent by a server
o A QUIC connection ID changes infrequently
o QUIC endpoints change the version they speak if they are sent a
Version Negotiation packet
o The version field in a QUIC long header is the same in both
directions
o Only one connection at a time is established between any pair of
QUIC endpoints
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